The increasing size and power output of turbine generators have led to higher losses in the ends of the laminated stator core. Unless appropriate design innovations are incorporated into this region, these higher losses will lead to increased temperatures. These augmented temperatures in the vicinity of the stator coil can cause a reduction in the life of the stator coil insulation. If not properly controlled by design, this can limit the range of leading power factor operation of large generators.
Historically, end iron heating was resolved in the stator tooth area by splitting the last few inches of core iron and by stepping the tooth end packs. Providing radial cooling ducts in the ends of axially ventilated stator cores has proven to be satisfactory for existing ratings and machine sizes. However, the combination of several slits and radial ducts in the end pack region of the stator has the adverse effect of mechanically weakening the core end structure.
Other schemes had utilized flux shields in the tooth portion of the stator core. These eddy current shields are typically copper plates or loops situated directly in front of the stator teeth. They are conductive members of low magnetic permeability arranged to provide circulating currents which divert stray flux away from the tooth region. For highly rated machines, the currents induced in this type of shield produce extremely high losses, many times higher than that normally seen in the stator end iron. In most cases, liquid cooling of the shields may be required.
Additional losses in the stator core end packs result from axial magnetic flux caused by the rotor and stator end winding currents. Under normal machine operation, the terminal voltage is produced by magnetic flux which travels radially across the air gap and is distributed uniformly over the length of the core. In the end regions, this flux fringes from the ends of the rotor body to the stator end packs. This fringing flux enters the end packs at right angles to the plane of the lamination of the core. The loss generated by the ensuing eddy currents is considerably higher than the losses due to the same flux density entering parallel to the laminations. The net fringing flux is a vector sum of both the rotor and the armature components, varying in magnitude with both load and power factor. This flux continues to penetrate axially into the end pack until it can turn radially and join the peripherally travelling main synchronous flux system.
Flux screens consisting of several laminated sheets of material which are located between finger plates and end plates of a generator to reduce the axial flux striking the end region of the generator core are disclosed in different shapes for example in U.S. Pat. No. 4,152,615 as well as U.S. Pat. No. 4,281,266. A different type of shield for the end of the laminated dynamical electric machine stator core to reduce penetration of the stray end region flux into the tooth portion of the stator core is disclosed in U.S. Pat. No. 3,731,127 and U.S. Pat. No. 3,886,387.
A further design of a flux shield is disclosed in DE 2638908. Here the flux shield is mounted in the generator leaving an air gap behind the flux shield allowing cooling air to circulate in this air gap. In order to increase the heat transfer from the flux shield to the cooling air specific aids are provided. One possibility indicated in this document is to provide protrusions on the backside of the flux shield around which the cooling air flows.